Photovoltaic cells or solar cells are photovoltaic components for direct generation of electrical current from sunlight. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years and continues to expand. Solar cells include a substrate, a back contact layer on the substrate, an absorber layer on the back contact layer, a buffer layer on the absorber layer, and a front contact layer above the buffer layer. The layers can be applied onto the substrate during a deposition process using, for example, sputtering and/or co-evaporation.
Semi-conductive materials are used in at least a portion of the absorber layer of some solar cells. For example, chalcopyrite based semi-conductive materials, such as copper indium gallium selenide (CIGS) (also known as thin film solar cell materials), are used to complete the formation of the absorber layer after the deposition process.
In semiconductor materials, the term “recombination,” refers to a phenomenon in which an electron recombines with a hole giving off excess energy to a second electron instead of emitting the energy as a photon. The second electron (and successive electrons) then give up the additional energy in a series of collisions, relaxing back to the edge of the band. Thus, the effect is a result of interactions between multiple particles, including multiple electrons and a hole. The net effect is that many electron-hole pairs, which could otherwise generate useful power, recombine, and the charge carriers are eliminated.
Because recombination is based on the ability of the charge carriers to exchange energy, the probability of recombination increases with a higher concentration of charge carriers.
In highly concentrated sunlight, recombination significantly reduces solar cell efficiency.